Many devices that operate in the Radio Frequency (RF) band utilize electronic components, such as inductors. Inductors can be used in analog portions of the RF circuits, such as Voltage Controlled Oscillators (VCOs). Moreover, as these types of products become more miniaturized, the inductors can be formed in the same semiconductor chip with the RF circuit rather than as a discreet component outside the semiconductor chip that includes the RF circuit.
FIG. 1A is a plan view of a conventional ribbon type inductor that is used in some RF devices. FIG. 1B is an equivalent schematic diagram of the ribbon type inductor, shown in FIG. 1A, that illustrates the parasitic capacitances associated with the ribbon type inductor. Because ribbon type inductors tend to be relatively large, these types of inductors may not be suitable for inclusion in highly integrated RF circuits in semiconductor chips.
FIG. 2 is a plan view of a conventional spiral type inductor that may be used in Complementary Metal-Oxide-Semiconductor (CMOS) circuits. Spiral type inductors are typically characterized by a large inductance due to the fact that the current flows in the same direction throughout the spiral, as shown by the arrows in FIG. 2. Spiral type inductors may also be characterized by small resistances due to the relatively small number of connections used to integrate the inductors with other components. The inductance provided by spiral type inductors may, therefore, be predictable. However, conventional spiral type inductors may also be characterized as having high-parasitic capacitance due to the coupling between the coil and the substrate on which the spiral type inductor is formed.
Another type of conventional inductor is a three-dimensional solenoid type inductor. In general, solenoid type inductors include coils (similar to the spiral inductors discussed above), but the coils are not coplanar. Rather, the coils are spaced apart from one another so that the inductor has a cylindrical shape, such as that associated with a solenoid. Solenoid type inductors may also be characterized as providing a relatively large inductance as the current flows in the same direction within the different coils of the inductor. Although three-dimensional solenoid type inductors can have low capacitance (due to relatively low coupling between the coils of the inductor and the substrate), such conventional inductors can be difficult to fabricate because these need many process steps.
As discussed above, inductors can be used in VCO circuits as part of RF devices such, as radio telephones (or cell phones). As is well known to those skilled in the art, conventional VCO circuits can utilize a symmetrical arrangement of components as shown in schematic circuit diagram of FIG. 3. Non-symmetry in the VCO circuit can cause unbalance of oscillation with regard to two oscillation terminals (out and/out). This degrades the coherency of the oscillation of the VCO circuit.
Inductors are also discussed in U.S. Patent Application No. US 20030030532 and in Japanese Patent Application JP2003-045712 the disclosures of which are incorporated herein by reference in their entireties.